Method and system for stabilized directional couplers

ABSTRACT

Methods and systems for stabilized directional couplers are disclosed and may include a system comprising first and second directional couplers formed by first and second waveguides, where one of the waveguides may comprise a length extender between the directional couplers. The directional couplers may be formed by reduced spacing between the waveguides on opposite sides of the length extender. An input optical signal may be communicated into one of the waveguides, where at least a portion of the input optical signal may be coupled between the waveguides in the first directional coupler and at least a portion of the coupled optical signal may be coupled between the waveguides in the second directional coupler. Optical signals may be communicated out of the system with magnitudes at a desired percentage of the input optical signal. The length extender may add phase delay for signals in one of the first and second waveguides.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application claims priority to U.S. Provisional Application61/797,692, filed on Dec. 13, 2012, which is hereby incorporated hereinby reference in its entirety.

FIELD

Certain embodiments of the invention relate to semiconductor processing.More specifically, certain embodiments of the invention relate to amethod and system for stabilized directional couplers.

BACKGROUND

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY

A system and/or method for stabilized directional couplers,substantially as shown in and/or described in connection with at leastone of the figures, as set forth more completely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chipcomprising grating couplers with perturbed waveguides, in accordancewith an example embodiment of the disclosure.

FIG. 1B is a diagram illustrating a CMOS chip, in accordance with anexample embodiment of the disclosure.

FIG. 1C is a diagram illustrating a CMOS chip coupled to an opticalfiber cable, in accordance with an example embodiment of the disclosure.

FIG. 2 is a schematic illustrating a directional coupler, in accordancewith an example embodiment of the disclosure.

FIG. 3A is a schematic illustrating a stabilized directional coupler, inaccordance with an example embodiment of the disclosure.

FIG. 3B illustrates Monte Carlo simulations for the design of astabilized 5% directional coupler, in accordance with an exampleembodiment of the disclosure.

FIG. 4A is a drawing illustrating a 1×3 stabilized directional coupler,in accordance with an example embodiment of the disclosure.

FIG. 4B illustrates experimental results for output power variation for1×3 splitters across a wafer, in accordance with an example embodimentof the disclosure

FIG. 5 is a drawing illustrating a stabilized 2×4 splitter tree, inaccordance with an example embodiment of the disclosure.

FIG. 6 is a drawing illustrating a stabilized monitoring tap, inaccordance with an example embodiment of the disclosure.

FIG. 7 is a drawing illustrating a Mach-Zehnder interferometer withstabilized directional couplers, in accordance with an exampleembodiment of the disclosure.

FIGS. 8A-8F illustrate the performance of stabilized directionalcouplers with optical test data collected on several wafers frommultiple lots, in accordance with an example embodiment of thedisclosure.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of the invention may be found in a method and system forstabilized directional couplers. Exemplary aspects of the invention maycomprise a system comprising first and second directional couplersformed by first and second waveguides, where one of the first and secondwaveguides may comprise a length extender between the first and seconddirectional couplers. The first and second directional couplers may beformed by reduced spacing between the first and second waveguides onopposite sides of the length extender. An input optical signal may becommunicated into one of the first and second waveguides, where at leasta portion of the input optical signal may be coupled between the firstand second waveguides in the first directional coupler and at least aportion of the coupled optical signal may be coupled between the firstand second waveguides in the second directional coupler. Optical signalsmay be communicated out of the system, where magnitudes of the opticalsignals communicated out of the system may be at a desired percentage ofthe input optical signal. The length extender may add phase delay forsignals in one of the first and second waveguides. The desiredpercentage may be based on the reduced spacing and a size of the lengthextender. The system may comprise a 2×2 or a 1×3 splitter. The systemmay comprise a 1/N splitter, where 1/N represents the signal strength ineach output of the 1/N splitter. A magnitude of the input optical signalmay be monitored utilizing a photodiode in a loop formed by one of thefirst or second waveguides. The optical signals communicated out of thesystem may be modulated before communicated to a second stabilizeddirectional coupler formed by the first and second waveguides andcomprising a second length extender. The optical signals may bemodulated utilizing phase modulators in each of the first and secondwaveguides. The system may be integrated in a Complementary Metal-OxideSemiconductor (CMOS) chip.

FIG. 1A is a block diagram of a photonically enabled CMOS chipcomprising grating couplers with perturbed waveguides, in accordancewith an exemplary embodiment of the invention. Referring to FIG. 1A,there is shown optoelectronic devices on a CMOS chip 130 comprisingoptical modulators 105A-105D, photodiodes 111A-111D, monitor photodiodes113A-113H, and optical devices comprising directional couplers103A-103K, optical terminations 115A-115D, and grating couplers117A-117H. There are also shown electrical devices and circuitscomprising amplifiers 107A-107D, analog and digital control circuits109, and control sections 112A-112D. The amplifiers 107A-107D maycomprise transimpedance and limiting amplifiers (TIA/LAs), for example.

Optical signals are communicated between optical and optoelectronicdevices via optical waveguides 110 fabricated in the CMOS chip 130.Single-mode or multi-mode waveguides may be used in photonic integratedcircuits. Single-mode operation enables direct connection to opticalsignal processing and networking elements. The term “single-mode” may beused for waveguides that support a single mode for each of the twopolarizations, transverse-electric (TE) and transverse-magnetic (TM), orfor waveguides that are truly single mode and only support one modewhose polarization is TE, which comprises an electric field parallel tothe substrate supporting the waveguides. Two typical waveguidecross-sections that are utilized comprise strip waveguides and ribwaveguides. Strip waveguides typically comprise a rectangularcross-section, whereas rib waveguides comprise a rib section on top of awaveguide slab.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of thecontinuous-wave (CW) laser input signal. The optical modulators105A-105D comprise high-speed and low-speed phase modulation sectionsand are controlled by the control sections 112A-112D. The high-speedphase modulation section of the optical modulators 105A-105D maymodulate a CW light source signal with a data signal. The low-speedphase modulation section of the optical modulators 105A-105D maycompensate for slowly varying phase factors such as those induced bymismatch between the waveguides, waveguide temperature, or waveguidestress and is referred to as the passive phase, or the passive biasingof the MZI.

The phase modulators may have a dual role: to compensate for the passivebiasing of the MZI and to apply the additional phase modulation used tomodulate the light intensity at the output port of the MZI according toa data stream. The former phase tuning and the latter phase modulationmay be applied by separate, specialized devices, since the former is alow speed, slowly varying contribution, while the latter is typically ahigh speed signal. These devices are then respectively referred to asthe LSPM and the HSPM. Examples for LSPM are thermal phase modulators(TPM), where a waveguide portion is locally heated up to modify theindex of refraction of its constituting materials, or forward biased PINjunction phase modulators (PINPM) where current injection into the PINjunction modifies the carrier density, and thus the index of refractionof the semiconductor material. An example of an HSPM is a reversedbiased PIN junction, where the index of refraction is also modulated viathe carrier density, but which allows much faster operation, albeit at alower phase modulation efficiency per waveguide length.

The outputs of the modulators 105A-105D may be optically coupled via thewaveguides 110 to the grating couplers 117E-117H. The directionalcouplers 103A-103K may comprise four-port optical couplers, for example,and may be utilized to sample or split the optical signals generated bythe optical modulators 105A-105D, with the sampled signals beingmeasured by the monitor photodiodes 113A-113H. The unused branches ofthe directional couplers 103D-103K may be terminated by opticalterminations 115A-115D to avoid back reflections of unwanted signals.

The directional coupler is one of the main building blocks in siliconphotonic circuits, and consists of two waveguides that are curved tocome in close proximity at a point so that light may couple evanescentlybetween the waveguides. Several types of directional couplers exist. Forexample, a 3 dB directional coupler splitter is a DC with two input andtwo output ports that can split the input light equally into the twooutputs, as illustrated by the directional couplers 103A-103C, forexample. It may be used in a splitter tree to create parallel channelsand in an MZI modulator, such as the modulators 105A-105D. Directionalcoupler taps may be used in a control system. These devices may tap offa predefined portion of the light from the main waveguide into a tapwaveguide that goes to a monitor photodiode, as illustrated by thedirectional couplers 103D-103K.

The performance of a directional coupler may be very sensitive todeviations in device dimensions, such as waveguide width/thickness andetch depth. Consequently, the coupling ratio, i.e. the fraction of lightcoupled over to the other waveguide, may differ significantly fromdirectional coupler to directional coupler due to process variations.This is undesirable, because it introduces imbalance between thechannels in a splitter tree, which results in a power penalty, and mayalso add uncertainty and potentially a power penalty for monitoringtaps.

In an example scenario, the directional couplers 103A-103K may comprisestabilized directional couplers, or stabilized splitters, where asplitter comprises two directional couplers in series. While thecoupling ratio of the same directional coupler design may differsignificantly from wafer to wafer and within one wafer, closely spaceddirectional couplers on one wafer may have strongly correlatedperformance. This correlation enables the design of stabilized taps andsplitters.

In an example scenario, the length of one waveguide, or arm, of thewaveguides between two directional couplers may be longer than the otherby incorporating a curved length extender, similar to a chicane in aracetrack, which may stabilize or reduce variation of light outputlevels between the outputs of directional couplers. The difference inwaveguide length needed to achieve stabilization may depend on thespecific correlation between the directional couplers and the tap ratioand may be determined through Monte Carlo simulations. This is shownfurther with respect to FIGS. 2-8.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D may be utilized to couple light received from opticalfibers into the CMOS chip 130, and the grating couplers 117E-117H may beutilized to couple light from the CMOS chip 130 into optical fibers. Thegrating couplers 117A-117H may comprise single polarization gratingcouplers (SPGC) and/or polarization splitting grating couplers (PSGC).In instances where a PSGC is utilized, two input, or output, waveguidesmay be utilized.

The optical fibers may be epoxied, for example, to the CMOS chip, andmay be aligned at an angle from normal to the surface of the CMOS chip130 to optimize coupling efficiency. In an example embodiment, theoptical fibers may comprise single-mode fiber (SMF) and/orpolarization-maintaining fiber (PMF).

In another exemplary embodiment, optical signals may be communicateddirectly into the surface of the CMOS chip 130 without optical fibers bydirecting a light source on an optical coupling device in the chip, suchas the light source interface 135 and/or the optical fiber interface139. This may be accomplished with directed laser sources and/or opticalsources on another chip flip-chip bonded to the CMOS chip 130.

The photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicatedto the amplifiers 107A-107D for processing. In another embodiment of theinvention, the photodiodes 111A-111D may comprise high-speedheterojunction phototransistors, for example, and may comprise germanium(Ge) in the collector and base regions for absorption in the 1.3-1.6 μmoptical wavelength range, and may be integrated on a CMOSsilicon-on-insulator (SOI) wafer.

The analog and digital control circuits 109 may control gain levels orother parameters in the operation of the amplifiers 107A-107D, which maythen communicate electrical signals off the CMOS chip 130. The controlsections 112A-112D comprise electronic circuitry that enable modulationof the CW laser signal received from the splitters 103A-103C. Theoptical modulators 105A-105D may require high-speed electrical signalsto modulate the refractive index in respective branches of aMach-Zehnder interferometer (MZI), for example. In an exampleembodiment, the control sections 112A-112D may include sink and/orsource driver electronics that may enable a bidirectional link utilizinga single laser.

In operation, the CMOS chip 130 may be operable to transmit and/orreceive and process optical signals. Optical signals may be receivedfrom optical fibers by the grating couplers 117A-117D and converted toelectrical signals by the photodetectors 111A-111D. The electricalsignals may be amplified by transimpedance amplifiers in the amplifiers107A-107D, for example, and subsequently communicated to otherelectronic circuitry, not shown, in the CMOS chip 130.

Integrated photonics platforms allow the full functionality of anoptical transceiver to be integrated on a single chip, the CMOS chip130, for example. A transceiver chip comprises optoelectronic circuitsthat create and process the optical/electrical signals on thetransmitter (Tx) and the receiver (Rx) sides, as well as opticalinterfaces that couple the optical signal to and from one or morefibers. The signal processing functionality may comprise modulating theoptical carrier, detecting the optical signal, splitting or combiningdata streams, and multiplexing or demultiplexing data on carriers withdifferent wavelengths. In another example scenario, a plurality of chipsmay be utilized, with an optical interposer for receiving electronicschips and photonics chips, in instances where the electronics chips andphotonics chips are manufactured in different CMOS nodes.

The light source may be external to the chip or may be integrated withthe chip in a hybrid scheme. It is often advantageous to have anexternal continuous-wave (CW) light source, because this architectureallows heat sinking and temperature control of the source separatelyfrom the transceiver chip 130. An external light source may also beconnected to the transceiver chip 130 via a fiber interface.

An integrated transceiver may comprise at least three opticalinterfaces, including a transmitter input port to interface to the CWlight source, labeled as CW Laser In 101; a transmitter output port tointerface to the fiber carrying the optical signal, labeled OpticalSignals Out; and a receiver input port to interface to the fibercarrying the optical signal, labeled Optical Signals In.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an exemplary embodiment of the invention. Referring to FIG. 1B,there is shown the CMOS chip 130 comprising electronic devices/circuits131, optical and optoelectronic devices 133, a light source interface135, CMOS chip front surface 137, an optical fiber interface 139, andCMOS guard ring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers, for example, that enable coupling of lightsignals via the CMOS chip surface 137, as opposed to the edges of thechip as with conventional edge-emitting devices. Coupling light signalsvia the CMOS chip surface 137 enables the use of the CMOS guard ring 141which protects the chip mechanically and prevents the entry ofcontaminants via the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theamplifiers 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the directionalcouplers 103A-103K, optical terminations 115A-115D, grating couplers117A-117H, optical modulators 105A-105D, high-speed heterojunctionphotodiodes 111A-111D, and monitor photodiodes 113A-113H.

In an example scenario, the directional couplers 103A-103K may comprisestabilized directional couplers. While the coupling ratio of the samedirectional coupler design may differ significantly from wafer to waferand within one wafer, closely spaced directional couplers on one wafermay have strongly correlated performance. This correlation enables thedesign of stabilized taps and splitters.

FIG. 1C is a diagram illustrating a CMOS chip coupled to an opticalfiber cable, in accordance with an exemplary embodiment of theinvention. Referring to FIG. 1C, there is shown the CMOS chip 130comprising the CMOS chip surface 137, and the CMOS guard ring 141. Thereis also shown a fiber-to-chip coupler 143, an optical fiber cable 145,and an optical source assembly 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the light source interface 135,the CMOS chip surface 137, and the CMOS guard ring 141 may be asdescribed with respect to FIG. 1B.

In an example embodiment, the optical fiber cable may be affixed, viaepoxy for example, to the CMOS chip surface 137. The fiber chip coupler143 enables the physical coupling of the optical fiber cable 145 to theCMOS chip 130.

In an example scenario, the directional couplers 103A-103K may comprisestabilized directional couplers. While the coupling ratio of the samedirectional coupler design may differ significantly from wafer to waferand within one wafer, closely spaced directional couplers on one wafermay have strongly correlated performance. This correlation enables thedesign of stabilized taps and splitters.

FIG. 2 is a schematic illustrating a directional coupler, in accordancewith an example embodiment of the disclosure. Referring to FIG. 2, thereis shown directional coupler 200 comprising waveguides 201A and 201B.The waveguides 201A and 201B may comprise silicon, for example, that hasbeen etched into ridge structures. The waveguides 201A and 201B maycomprise a dielectric cladding layer or air cladding, for example.

In operation, light may be coupled into one waveguide, as indicated bythe arrow pointing in to the waveguide 201A. Due to the bends in thewaveguides 201A and 201B, bringing them close together, light may beevanescently coupled into the waveguide 201B. The amount of lightcoupled to the waveguide 201B may be configured by the distance betweenthe waveguides in the directional coupler 200. It should be noted thatlight may be coupled into and out of any end of the waveguides, and thedirectional coupler 200 may be bi-directional in that light can travelin both directions.

The performance of the directional coupler 200 may be very sensitive todeviations in device dimensions, such as waveguide 201A and 201Bwidth/thickness and etch depth. Consequently, the coupling ratio, i.e.the fraction of light coupled over to the other waveguide, may differsignificantly from directional coupler to directional coupler due toprocess variations. This is undesirable, because it introduces imbalancebetween the channels in a splitter tree, which results in a powerpenalty, and may also add uncertainty and potentially a power penaltyfor monitoring taps. This imbalance may be mitigated and/or eliminatedby utilizing a stabilized directional coupler as illustrated in FIGS.3-8.

FIG. 3A is a schematic illustrating a stabilized directional coupler, inaccordance with an example embodiment of the disclosure. Referring toFIG. 3A, there is shown stabilized directional coupler 300 comprisingwaveguides 301A and 301B that come in close proximity in two locations,thus forming the directional couplers 303A and 303B. In addition, thewaveguide 301B may comprise a length extender 305, which may comprise acurved region added to the design of the waveguide 301B to provide alonger optical path. Therefore, the stabilized directional coupler 300comprises a 2×2 splitter formed by the combination of the twodirectional couplers 303A and 303B with the length extender 305.

In an example scenario, a stabilized X % directional coupler may beconfigured by cascading two directional couplers, the first one,directional coupler 303A with a targeted 100% coupling, and the secondone, directional coupler 303B, with a targeted 100−X % coupling, wherethe length extender 305 provides a difference in length/phase delay inthe connecting waveguides 301A and 301B. The 3 dB splitter is a specialcase of the X % tap, where X=50%.

The difference in waveguide length provided by the length extender 305in one of the waveguides to achieve stabilization depends on thespecific correlation between the directional couplers 303A and 303B andthe tap ratio, and may be determined through Monte Carlo simulations. Itshould be noted that the position of the 100% and 100−X % directionalcoupler is interchangeable.

The simulation for determining the size of the length extender 305 mayproceed as follows. In an example scenario, the waveguides 301A and 301Bmay be approximately 360 nm wide, and the coupling ratios of 100% and100−X % may be targeted by adjusting the minimum gap between thewaveguides 301A and 301B.

For light coming in from one input waveguide we can write the inputamplitudes as:

${input} = \begin{bmatrix}1 \\0\end{bmatrix}$The transfer matrices for the directional couplers may be written as:

$T_{D\; C\; 1} = {\begin{bmatrix}{\cos\;\phi_{1}} & {{- i}\;\sin\;\phi_{1}} \\{{- i}\;\sin\;\phi_{1}} & {\cos\;\phi_{1}}\end{bmatrix}\mspace{14mu}{and}}$ $T_{D\; C\; 2} = \begin{bmatrix}{\cos\;\phi_{2}} & {{- i}\;\sin\;\phi_{2}} \\{{- i}\;\sin\;\phi_{2}} & {\cos\;\phi_{2}}\end{bmatrix}$

with (sin ϕ₁)²=CR₁ and (sin ϕ₂)²=CR₂, where CR₁ and CR₂ are the couplingratios of the two directional couplers. For example, for a 100%directional coupler,

${\phi_{1} = \frac{\pi}{2}};$for a 95% DC, ϕ₁≈1.345.

The transfer matrix for the center section, or length extender, thatintroduces the phase difference δ from a difference in waveguide lengthmay be represented by:

$T_{\Delta\; L} = {\begin{bmatrix}1 & 0 \\0 & {\exp\left( {i\;\delta} \right)}\end{bmatrix}.}$

The output amplitudes may then be calculated from:output=T_(DC 2)T_(ΔL)T_(DC 1)input

Values for ϕ₁ may be randomly generated from a probability distributionthat may be established based on experimental test data for single-stagedirectional couplers, and may be approximated as a normal distribution.For a certain value of ϕ₁, ϕ₂ may be set based on the correlationbetween the two variables. This correlation may also be establishedbased on experimental test data for single-stage directional couplers.The expected distribution of coupling ratios may be calculated fordifferent values of δ. The results of Monte Carlo simulations (assumingperfect correlation between the 100% and 95% DCs) for a 5% stabilized DCare shown in FIG. 3B.

FIG. 3B illustrates Monte Carlo simulations for the design of astabilized 5% directional coupler, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 3B, the upper plot showsthe coupling ratio between outputs of a stabilized directional couplerversus δ, which comprises the phase difference from a difference inwaveguide length. It can be observed that the lowest variability occursin a range near δ=3+2nπ radians, with n being an integer.

The lower plot illustrates the standard deviation, in percent, of thecoupling ratios versus δ again showing the lowest variability occurringin a range near δ=3+2nπ radians, with n being an integer.

FIG. 4A is a drawing illustrating a 1×3 stabilized directional coupler,in accordance with an example embodiment of the disclosure. Referring toFIG. 4A there is shown 1×3 stabilized directional coupler 400 comprisingwaveguides 401A-401C and length extenders 405A and 405B. Therefore, the1×3 stabilized directional coupler 400 comprises a 1×3 splitter formedby the cascaded 1×3 directional couplers 403A and 403B that may beformed by the reduced spacing between the waveguides 401A-401C.

A stabilized 1×3 coupler, or 1×3 splitter, may be formed by cascadingtwo 1×3 directional couplers and introducing a difference in waveguidelength between center and outer waveguides, as illustrated by the lengthextenders 405A and 405B. In an example scenario, the first directionalcoupler 403A may be targeted at coupling all light from the center tothe outer waveguides 401A and 401C and the second directional coupler403B may be targeted at coupling ⅓ of the input light back to the centerwaveguide 403B. In this manner, the output signals, Light Out A-C mayeach comprise ⅓ of the input signal, namely Light In shown in FIG. 4A.

FIG. 4B illustrates experimental results for output power variation for1×3 splitters across a wafer, in accordance with an example embodimentof the disclosure. Referring to FIG. 4B, there is shown a plot ofexperimental data for a single stage 1×3 directional coupler, i.e.,without length extenders, and a stabilized directional coupler.

The left plot shows the average output powers in the center outputwaveguide for single-stage and stabilized directional couplers across awafer. The box plots show that the stabilized 1×3 directional couplersshow significantly less variation across the wafer than the single stage1×3 splitter.

Similarly, the right plot shows the distribution of average power outputacross the wafer, with significantly more variation for the single-stage1×3 splitter versus the stabilized directional coupler shown by the dataon the right-most plot in FIG. 4B.

FIG. 5 is a drawing illustrating a stabilized 2×4 splitter tree, inaccordance with an example embodiment of the disclosure. Referring toFIG. 5, there is shown 2×4 splitter tree 500 comprising waveguides501A-501D and length extenders 505A-505C, which may be utilized to formstabilized 2×2 directional couplers 510, 520, and 530. Each of thestabilized 2×2 directional couplers 510, 520, and 530 may comprise apair of directional couplers, e.g., directional coupler pairs 503A/503B,503C/503D, and 503E/503F, and length extenders 505A-505C, respectively.Accordingly, the serial/parallel connection of the stabilized 2×2directional couplers 510, 520, and 530 may result in equal output powersfor Light Out A-D when each of the 2×2 directional couplers 510, 520,and 530 comprise 50% (3 dB) directional couplers.

In an example scenario, any number of output directional couplers may beformed by combining 2×2 and 1×3 stabilized directional couplers, limitedby useful output power and available chip area, for example. Forexample, 1/N splitters, or splitter trees, may be formed with N being aninteger and 1/N representing the signal intensity at each of the outputsof the splitter.

FIG. 6 is a drawing illustrating a stabilized monitoring tap, inaccordance with an example embodiment of the disclosure. Referring toFIG. 6, there is shown stabilized monitoring tap 600 comprising astabilized directional coupler 610 and a photodiode 607. The stabilizeddirectional coupler 610 may comprise a 2×2 stabilized directionalcoupler with waveguides 601A and 601B, directional couplers 603A and603B, and a length extender 605.

The photodiode 607 may comprise a waveguide photodiode that is connectedas shown in FIG. 6 such that it may allow monitoring light coming fromboth directions, as illustrated by Light In/Out at both ends of thewaveguide 601A.

In an another example scenario, a grating coupler may be integrated inplace of the photodiode 607 on the chip comprising the stabilizedmonitoring tap 600 so that light may be detected off-chip.

FIG. 7 is a drawing illustrating a Mach-Zehnder interferometer withstabilized directional couplers, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 7, there is shown astabilized Mach Zehnder Interferometer (MZI) 700 comprising waveguides701A and 701B, 2×2 stabilized directional couplers 710 and 720, andphase modulators 707A and 707B.

The 2×2 stabilized directional couplers 710 and 720 may comprisedirectional couplers 703A/703B and 703C/703D, and length extenders 705Aand 705B, respectively. In instances where the 2×2 stabilizeddirectional couplers 710 and 720 comprise stabilized 3 dB splitters, thestabilized couplers may be used to improve the performance of an MZImodulator as shown in FIG. 6, by reducing the optical modulationamplitude (OMA) penalty, as compared to a single directional coupler.

FIGS. 8A-8F illustrate the performance of stabilized directionalcouplers with optical test data collected on several wafers frommultiple lots, in accordance with an example embodiment of thedisclosure. For all cases, the stabilized directional couplers show asignificant reduction in variability compared to single-stage designs.The remaining variability in the stabilized directional coupler resultsis in part due to test gauge and imperfect correlation between thedirectional couplers.

FIGS. 8A and 8B show experimental results for 1% and 2% designs,comparing results between single stage and stabilized directionalcouplers, where the stabilized structures show coupling ratiopercentages of near 1% and 2%, respectively, compared to the singlestage directional couplers that result in 1.2% and 2.3%, respectively,and with significantly higher variability across the wafer for thesingle stage directional couplers, as shown by the distribution plots tothe right.

Similarly, FIGS. 8C and 8D show experimental results for 5% and 10%designs, comparing results between single stage and stabilizeddirectional couplers, where the stabilized structures show couplingratio percentages of 5.1% and 10.4%, respectively, compared to thesingle stage directional couplers that result in 5.5% and 10.5%,respectively, and with significantly higher variability across the waferfor the single stage directional couplers, as shown by the distributionplots to the right.

FIG. 8E shows experimental results for a 50% directional coupler design,comparing results between a single stage and a stabilized directionalcoupler, where the stabilized structures show coupling ratio percentagesof near 50% compared to the single stage directional coupler thatresults in 54.5% with significantly higher variability across the waferfor the single stage directional couplers, as shown by the distributionplot to the right.

Table 1 summarizes the performance improvement for the stabilized DCdesigns, showing the standard deviation of coupling ratio forsingle-stage and stabilized directional coupler designs.

Target Single-stage Stabilized coupling DC standard DC standardReduction in ratio deviation deviation variability 1% 0.29% 0.12% 2.4 2%0.49% 0.15% 3.3 5% 0.99% 0.27% 3.7 10%  1.59% 0.48% 3.3 50%  4.11% 1.21%3.4

FIG. 8F shows experimental results for a stabilized 100% directionalcoupler, which may be used as an alternative to a waveguide crossing.Waveguide crossings add flexibility for routing light on a chip. Adirectional coupler with a coupling ratio of 100% may be used as analternative for a waveguide crossing. When a 100% directional coupler isused as a waveguide crossing, the small amount of the light that is notcoupled over (due to small variations in device dimensions) may beconsidered crosstalk. A stabilized 100% directional coupler, consistingof a 100% directional coupler

$\left( {\phi_{1} = \frac{\pi}{2}} \right)$and a 0% directional coupler (ϕ₂=π) may reduce crosstalk, as illustratedin FIG. 8F. The 0% directional coupler used in this example embodimentis a directional coupler with the correct waveguide spacing and lengthso that all light is nominally coupled over to the other waveguide andthen back, resulting in a coupling ratio of 0%.

In an example embodiment, a method and system are disclosed forstabilized directional couplers. In this regard, aspects of theinvention may comprise by first and second waveguides, where one of thefirst and second waveguides may comprise a length extender between thefirst and second directional couplers, and the first and seconddirectional couplers may be formed by reduced spacing between the firstand second waveguides on opposite sides of the length extender. An inputoptical signal may be communicated into one of the first and secondwaveguides, at least a portion of the input optical signal may becoupled between the first and second waveguides in the first directionalcoupler and at least a portion of the coupled optical signal may becoupled between the first and second waveguides in the seconddirectional coupler.

Optical signals may be communicated out of the stabilized directionalcoupler, where magnitudes of the optical signals communicated out of thestabilized directional coupler may be at a desired percentage of theinput optical signal. The length extender may add phase delay forsignals in the one of the first and second waveguides. The desiredpercentage may be based on the reduced spacing and a size of the lengthextender. The system may comprise a 2×2 or a 1×3 splitter.

The system may comprise a 1/N splitter, where 1/N represents the signalstrength at each output of the splitter. A magnitude of the inputoptical signal may be monitored utilizing a photodiode in a loop formedby one of the first or second waveguides. The optical signalscommunicated out of the stabilized directional coupler may be modulatedbefore communicating the modulated optical signals to a secondstabilized directional coupler formed by the first and second waveguidesand comprising a second length extender. The optical signals may bemodulated utilizing phase modulators in each of the first and secondwaveguides. The stabilized directional coupler may be integrated in aComplementary Metal-Oxide Semiconductor (CMOS) chip.

As utilized herein, “and/or” means any one or more of the items in thelist joined by “and/or”. As an example, “x and/or y” means any elementof the three-element set {(x), (y), (x, y)}. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, a device/module/circuitry/etc. is “operable” to performa function whenever the device/module/circuitry/etc. comprises thenecessary hardware and code (if any is necessary) to perform thefunction, regardless of whether performance of the function is disabled,or not enabled, by some user-configurable setting.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method for communication, the methodcomprising: in a system comprising first and second directional couplersthat comprise first and second waveguides, wherein the first waveguidecomprises a length extender increasing a length of the first waveguidewith respect to the second waveguide between said first and seconddirectional couplers, wherein the length extender has a length thatresults in a phase difference of 3+2*n*pi where n is an integer, andwherein said first and second directional couplers comprise reducedspacing between said first and second waveguides on opposite sides ofsaid length extender and the first directional coupler has a coupling ofX percent while the second directional coupler has a coupling of 100−Xpercent: communicating an input optical signal into one of said firstand second waveguides; coupling at least a portion of said input opticalsignal between said first and second waveguides in said firstdirectional coupler and coupling at least a portion of the coupledoptical signal between said first and second waveguides in said seconddirectional coupler; and communicating optical signals out of saidsystem at a desired percentage of said input optical signal.
 2. Themethod according to claim 1, wherein said length extender adds phasedelay for signals in said one of said first and second waveguides. 3.The method according to claim 1, wherein said desired percentage isbased on said reduced spacing and a size of said length extender.
 4. Themethod according to claim 1, wherein said system comprises a 2×2splitter.
 5. The method according to claim 1, wherein said systemcomprises a 1×3 splitter.
 6. The method according to claim 1, whereinsaid system comprises a 1/N splitter with N outputs, wherein 1/Nrepresents a fractional output intensity at each output of the 1/Nsplitter.
 7. The method according to claim 1, comprising monitoring amagnitude of said input optical signal utilizing a photodiode in a loopcomprising one of said first or second waveguides.
 8. The methodaccording to claim 1, comprising modulating said optical signalscommunicated out of said system before communicating said modulatedoptical signals to a second system comprising said first and secondwaveguides and a second length extender.
 9. The method according toclaim 8, wherein said optical signals are modulated utilizing phasemodulators in each of said first and second waveguides.
 10. The methodaccording to claim 1, wherein said system is integrated in aComplementary Metal-Oxide Semiconductor (CMOS) chip.
 11. A system forcommunication, the system comprising: first and second directionalcouplers that comprise first and second waveguides, wherein the firstwaveguide comprises a length extender increasing a length of the firstwaveguide with respect to the second waveguide between said first andsecond directional couplers, wherein the length extender has a lengththat results in a phase difference of 3+2*n*pi where n is an integer,and wherein said first and second directional couplers are formed byreduced spacing between said first and second waveguides on oppositesides of said length extender and the first directional coupler has acoupling of X percent while the second directional coupler has acoupling of 100−X percent, said system being operable to: receive aninput optical signal into one of said first and second waveguides;couple at least a portion of said input optical signal between saidfirst and second waveguides in said first directional coupler and coupleat least a portion of the coupled optical signal between said first andsecond waveguides in said second directional coupler; and communicateoptical signals out of said system at a desired percentage of said inputoptical signal.
 12. The system according to claim 11, wherein saidlength extender adds phase delay for signals in said one of said firstand second waveguides.
 13. The system according to claim 11, whereinsaid desired percentage is based on said reduced spacing and a size ofsaid length extender.
 14. The system according to claim 11, wherein saidsystem comprises a 2×2 splitter.
 15. The system according to claim 11,wherein said system comprises a 1×3 splitter.
 16. The system accordingto claim 11, wherein said system comprises a 1/N splitter with Noutputs, wherein 1/N represents a fractional output intensity at eachoutput of the 1/N splitter.
 17. The system according to claim 14,wherein a photodiode in a loop comprising one of said first or secondwaveguides is operable to monitor a magnitude of said received opticalsignal.
 18. The system according to claim 11, wherein said system iscoupled to phase modulators in said first and second waveguides and asecond system comprising said first and second waveguides and comprisinga second length extender, wherein said phase modulators are operable tomodulate said optical signals communicated out of said system beforesaid optical signals are communicated to said second system.
 19. Thesystem according to claim 11, wherein said system is integrated in aComplementary Metal-Oxide Semiconductor (CMOS) chip.
 20. A system forcommunication, the system comprising: a 1×3 directional couplercomprising first and second directional couplers comprising first,second, and third waveguides, wherein: said first waveguide comprises afirst length extender and said third waveguide comprises a second lengthextender; said first and second length extenders being located betweensaid first and second directional couplers, each length extender havinga length that results in a phase difference of 3+2*n*pi where n is aninteger; and said first and second directional couplers comprise reducedspacing between said first waveguide and said second waveguide andreduced spacing between said third waveguide and said second waveguideon opposite sides of said first and second length extenders and saidfirst directional coupler has a 100 percent coupling from the secondwaveguide to the first and third waveguides and the second directionalcoupler has a 33 percent coupling of the input light back to the secondwaveguide, said directional coupler being operable to: receive an inputoptical signal into said second waveguide; couple at least a portion ofsaid input optical signal between said second waveguide and said firstwaveguide in said first directional coupler and couple at least aportion of the coupled optical signal between said second waveguide andsaid third waveguide; and couple at least a portion of said coupledoptical signals in said first and third waveguides back to said secondwaveguide in said second directional coupler such that optical signalscommunicated out of said 1×3 directional coupler are each the same assaid input optical signal but each are at equal magnitudes that are ⅓the magnitude of said input optical signal.